(1) Field of the Invention
The present invention relates to an image forming apparatus using an electrophotographic process or the like, such as a copier, printer, facsimile machine, etc., in particular, relating to a technique for converting the resolution of the image in the image forming apparatus and a method for improving the practical resolution hence to make the reproduced image more distinct.
(2) Description of the Prior Art
In image forming apparatuses, typically, using an electrophotographic process, the surface of the photoreceptor as a recording medium is irradiated with light in accordance with an image to be formed. Static charge on the surface of the photoreceptor which has been previously uniformly electrified is discharged from the areas which are irradiated with light, thus the remaining charge forms a static latent image. This static latent image is visualized with toner as the coloring matter, forming a toner image, which in turn, is transferred to a properly fed, sheet-like transfer material, etc. This transfer material is made to pass through the fixing unit so that the toner image carried thereon may be permanently retained on the transfer material, and then this is discharged from the image forming apparatus.
As the means for radiating the light image onto the photoreceptor in accordance with an image to be formed, a semiconductor laser or the like, which is controlled as to its light radiation in accordance with the image information, is used so that the light beam illuminates the surface of the photoreceptor with an image to be formed. Accordingly, the light image formed by the light beam in accordance with the image information is made up of selected pixels.
In such an configuration, no problem occurs in creating an image if the input image information is composed of simple horizontal and/or vertical lines. However, when an image containing slanted lines etc. needs to be created, pixel spacing and jaggies are formed causing degradation. To deal with this, in order to eliminate jaggies and the like, conventionally it is checked whether the edges of image elements are inclined, and if an image element has an inclined portion, the pixels are interpolated or modified in their shape to eliminate the jaggies etc. to the eye, improving the practical resolution.
For example, it is disclosed in the specification of U.S. Pat. No. 5,005,139, that jaggies etc., at image edges or slanted portions are adapted to be eliminated by checking whether the edges of the input image are formed of slanted lines and curved lines and then the dots of recording pixels within the detected portions are changed in size, by adding extra dots or by deleting some dots to thereby eliminate jaggies etc., at the image edges and slanted portions.
In accordance with the above U.S. Patent disclosure, jaggies at the image edges, etc., can be eliminated by checking edges or curved lines (including slanted lines etc.) in the image information of pictorial images and text images etc., recognizing the pixel arrangement of the peripheral pixels, those to the top, bottom, left and right of the detected area, judging the balance of the image based on the pixel arrangement and modifying the size and shape of the recording pixels of the detected area, thus smoothening the image. Therefore, the reproduced image will be improved, being practically equivalent to that produced by a high-resolution process, thus providing more markedly clear and distinct images.
However, in accordance with the technique of the U.S. patent publication, it is necessary to provide a means for detecting edges and curved lines in the image as well as providing a plurality of storage (a plurality of registers etc.,) in order to refer the pixel arrangement of the peripheral pixels around the observed pixel as a recording target. Further, in order to recognize the condition of the peripheral pixels held in the storage, a reference pattern storage or the like for previously holding a large number of reference states of the peripheral pixels is needed. Thus, the circuit configurations unavoidably become very complicated. Moreover, an increased processing time for recognizing edges etc., in the image, inclusive of the processing time for comparing the detected pattern with a large number of reference patterns, is needed, resulting in an impossibility to perform high-speed processing.
As stated above, in order to recognize the peripheral conditions, i.e., those to the top, bottom, left and right of the observed pixel, it is necessary to provide a storage of high capacity for a buffer for temporarily storing the input image information and the memory for storing a large number of reference patterns, increasing the cost over and above that of the complicated circuit configurations.
There is another technique which simply divides each pixel of the image information into a prescribed number of parts. Even in such a case, it is impossible to eliminate jaggies etc., by dividing the pixels. Therefore, when the sub-pixels (the divided pixels) are made black or white, the conditions of the upper, lower, left and right pixels around the observed pixel are checked in the same manner as above. Therefore, this case also results in complicated circuit configurations.